For a PN-junction varactor in a current BiCMOS process, an N-terminal, i.e. an N-type region of the PN-junction, is generally picked up through a buried layer or a well situated at the bottom of a shallow trench field oxide region, namely a shallow trench isolation (STI), and an active area connected to the buried layer or the well. This method is determined by the vertical structure of the existing PN-junction varactor. The existing PN-junction varactor has disadvantages such as a large device area and a large contact resistance. Moreover, as an additional active area must be arranged to pick up the N-terminal, and must be isolated from a P-terminal by STIs or other field oxide regions, further reduction of the device size is greatly limited.